As an abrasive for finely polishing an optical glass, a glass substrate or the like in a finishing process, fine particles with high degree of hardness have been used. Such fine particles can be typified by fine particles of diamond, boron nitride, silicon carbide, alumina, alumina-zirconia, zirconium oxide and cerium oxide.
Some of the main components of the abrasives are obtained from minerals that are not produced in Japan, and thus partially relies on imported materials. In addition, many of the main components of the abrasives are expensive.
The abrasive used for the above purposes is composed of fine particles with high degree of hardness and thus important for uses as an optical abrasive for electronic components such as optical lens, semiconductor silicone substrates and glass plates of liquid crystal displays in large quantity. An abrasive for optical abrasion contains the above-mentioned compound as its main component, and may further contain a transition metal element(s) such as sodium salts and chromium salts and/or a rare-earth element(s) such as yttrium and dysprosium. Thus, an abrasive for optical abrasion is strictly inhibited from being simply disposed considering environmental pollution. A waste liquid of an abrasive used in polishing is therefore strongly desired to be made non-polluting. Hence, techniques to reuse a resource(s) in an optical abrasive waste liquid containing a used abrasive and to make such a waste liquid non-polluting are important.
In various fields of industry, a conventional method for disposing a waste liquid that contains suspended particles normally includes aggregating and separating the suspended particles using a neutralizer, inorganic coagulant or polymeric coagulant, discharging a treated solution and disposing the aggregated and separated sludge by incineration or the like.
The abrasive for the above purposes is used in large quantity in a polishing process, and its waste liquid contains a component(s) derived from the polished object, e.g., debris of a polished optical glass. In addition, it is difficult to efficiently separate the abrasive from the component(s) derived from the polished object. Because a waste liquid is disposed after use at present as described above, there are problems concerning environmental burdens and disposal cost.
Thus, it has become important to establish a method for efficiently collecting a main component of an abrasive for recycling the element which is a scarce material.
As a method for collecting an abrasive component, Patent Document 1 discloses a solid-liquid separation method including adding an electrolyte to a used abrasive liquid to aggregate and let settle out the abrasive and to dissolve a component(s) derived from a polished substrate. In a method described in Patent Document 1, an alkali metal hydroxide, alkali metal carbonate, alkali metal salt and ammonium salt are used as the electrolyte.
Patent Document 2 discloses a solid-liquid separation method including adding a sodium hydroxide solution and potassium hydroxide solution to a used abrasive and separating a solid component. Patent Document 3 discloses a method including adding sulfuric acid to a used abrasive, applying heat thereto, dissolving a rare-earth element and rare metal, and separating and removing an aggregate of silica etc. in the slurry.
Patent Document 4 discloses a method for collecting a colloidal silica-based abrasive, the method including adding an alkali to a chemical mechanical polishing (CMP) waste liquid in the presence of a magnesium ion to adjust pH to 10 or higher and cause aggregation. Non-Patent Document 1 is a review article regarding the above-described metal-collecting methods.
However, the above methods disclosed by Patent Documents 1 to 4 provide a collected abrasive of insufficient purity. Such a collected abrasive is not suitable for fine polishing.
In the method of Patent Document 4, if an abrasive mainly composed of cerium oxide is used in polishing an object mainly composed of silicon such as a glass, addition of an additive such as magnesium chloride to an abrasive-containing slurry that contains a used abrasive at a pH of 10 or higher causes co-aggregation of an abrasive component and a glass component, which lowers the purity of an obtained regenerated abrasive. This would be because when the pH is 10 or higher, a component(s) derived from a polished object mainly composed of silicon (e.g., glass) become easier to aggregate than the abrasive component does upon addition of the additive.
Patent Document 5 discloses a method for producing a regenerated cerium oxide-containing abrasive by freezing a collected used liquid to regenerate secondary particles of cerium oxide and removing water. However, this method of Patent Document 5 requires huge facilities for conducting the freezing and thus requires enormous initial investment.